Tree Rings and Observations Suggest No Stable Cycles in Sierra Nevada Cool‐Season Precipitation

Abstract

In California there is often too much or too little water. This was exemplified recently by the need for water-use restrictions during the severe drought from 2012 to 2016, followed immediately by a water surplus in water-year 2016-2017 that nearly caused northern California's Oroville Dam to overflow (Wang et al., 2017). Despite over a century of record keeping and management strategies that address this variability, the volatility of precipitation in California continues to pose major challenges to water resource managers. Management must minimize effects of drought on water supplies while also keeping reservoir levels low enough to minimize flood risk (Dettinger et al., 2011; Hanak et al., 2017; Stewart et al., 2020). California's exposure to hydrological volatility is particularly high because of its geographic setting: the majority of the state's surface water supply falls in the Sierra Nevada during the relatively short cool season of November-March (Dettinger, 2013; Dettinger et al., 2011, 2018), giving this region and season particular importance to California's water resources. While parts of the state, especially southern California, have worked to diversify water portfolios with a combination of surface, imported, and ground water, and to employ conservation measures and water markets (Gottlieb & FitzSimmons, 1991; Lund et al., 2018), California remains vulnerable to extreme variability of precipitation (Swain et al., 2018). Extreme hydrological shortages in the Sierra Nevada can be only moderately buffered by water supplies in regions and seasons not experiencing drought. California's drought and flood risk is amplified by the limited reliability of seasonal forecasts of Sierra Nevada precipitation (Jones et al., 2015). While forecast skill beyond a 2-week horizon is inherently poor globally, seasonal forecasting for parts of the western United States is aided by a teleconnection to the El Niño-Southern Oscillation (ENSO) (Huang et al., 2019; Seager & Hoerling, 2014). This is not the case, however, for the Sierra Nevada, which span the neutral point of the ENSO dipole that often drives Abstract California's water resources rely heavily on cool-season (November-March) precipitation in the Sierra Nevada. Interannual variability is highly volatile and seasonal forecasting has little to no skill, making water management particularly challenging. Over 1902-2020, Sierra Nevada cool-season precipitation totals exhibited significant 2.2-and 13-15-year cycles, accounting for approximately 40% of total variability and perhaps signifying potential as seasonal forecasting tools. However, the underlying climate dynamics are not well understood and it is unclear whether these cycles are stable over the long term. We use tree rings to reconstruct Sierra Nevada cool-season precipitation back to 1400. The reconstruction is skillful, accounting for 55%-74% of observed variability and capturing the 20th-century 2.2-and 13-15-year cycles. Prior to 1900, the reconstruction indicates no other century-long periods of signif...

Reconstructing Extreme Precipitation in the Sacramento River Watershed Using Tree‐Ring Based Proxies of Cold‐Season Precipitation

“…Our reconstruction is a direct extension of the SPI reconstruction developed by Williams et al. (2020a) and is similar to one produced by Stahle et al. (2020), but ours (and that of Williams et al., 2020a) incorporates a number of new RWI chronologies, including new precipitation‐sensitive chronologies in the northern Sierra Nevada (Lepley et al., 2020).…”

“…This result underscored the possibility that precipitation-sensitive tree-ring chronologies from these regions might be useful in reconstructing SRW extreme precipitation events. To test this, we first created a gridded reconstruction of cold-season SPI across the West, which is an extension of the reconstruction presented in Williams et al (2020a). This gridded reconstruction filtered the treering chronologies into a spatially and temporally continuous data set useful for the reconstruction of SRW precipitation extremes, and serves as a stand-alone product useful for understanding past cold-season precipitation variability across the West.…”

“…The SPI expresses cold-season precipitation totals as standardized anomalies after the data have been normalized. Our reconstruction is a direct extension of the SPI reconstruction developed by Williams et al (2020a) and is similar to one produced by Stahle et al (2020), but ours (and that of Williams et al, 2020a) incorporates a number of new RWI chronologies, including new precipitation-sensitive chronologies in the northern Sierra Nevada (Lepley et al, 2020). Our reconstruction also retains a higher effective spatial resolution (described below), which is important to our goal of detecting nuanced spatial signatures of regional precipitation events.…”

“…Our reconstruction differs from that in Williams et al. (2020a) because we allow for multiple calibration periods to be considered; Williams et al. (2020a) only used 1902–2000.…”

“…For the cold‐season precipitation reconstruction we use the same network of western North American tree‐ring records used by Williams et al. (2020a), which is an update of the network used for previous hydroclimate reconstructions in western North America (Cook et al., 2010b; Stahle et al., 2020; Williams et al., 2020b). The network is composed of 1,620 chronologies of ring‐width index (RWI) from the western North American domain of 24°N‐56°N, 102°W‐124°W and have continuous data coverage for at least 1800–1983.…”